Pipeline construction and laying is known to be carried out using laying vessels, which are equipped with production lines configured to construct the pipelines and laying devices configured to lay them. Pipeline construction and laying is carried out according to two chief methods, which are chosen depending on the depth of the seabed.
The first method, suitable for deep-water laying, contemplates making a pipeline using a substantially vertical production line and laying the underwater pipeline in a substantially vertical position, such that the pipeline assumes a J-shape in the section extending between the laying vessel and the bed of the body of water.
The second method, particularly suitable for laying in shallow to medium depth waters, contemplates making the underwater pipeline using a substantially horizontal production line and laying the underwater pipeline by a curved supporting structure, which has the function of guiding and supporting the underwater pipeline along a curved path that has a first part above the body of water, and a second part in the body of water. Underwater pipelines laid according to the second method assume an S-shape between the laying vessel and the bed of the body of water.
Pipelines of the above-indicated type comprise parts, such as valves for example, which radially protrude from the cylindrical profile of the pipelines. These parts are generally known as “special parts” or “bulky items”, and require that the laying devices are configured to enable for their passage.
One known problem associated with laying pipelines with special parts is the pipeline rotating about its longitudinal axis while the pipeline is being laid. There can be various causes of pipeline rotation. In the J-lay configuration, the pipeline is mainly subjected to traction, and rotation can be caused by the action of a cross-current. The special part, which usually has rather relatively large cross-sections, tends to take a position that offers least resistance to the current, thereby inducing a localized twisting moment on the pipe that cancels out along the laying line. This twisting moment has the effect of producing a rotation of the part that grows as the depth of the bed of the body of water increases.
In the S-lay configuration, the bending moment caused by the curving of the pipeline during the laying stage brings the outermost fibres of the pipeline to a plastic state. Once the curved section of pipeline takes its rectilinear shape again and the state of traction reverts to prevailing over the state of flexure along the plasticized section, the redistribution of stress along the directrix of the pipeline becomes non-homogeneous due to the residual tension and causes torsional instability. In this condition, the pipeline turns around its axis to find a new configuration of equilibrium.
The rotation induced by the action of the currents on the special parts, and described with reference to J-laying, adds to this effect.
In any case, a special part has a tendency to turn about the axis of the pipeline according to a helical trajectory. Since special parts must be laid on the bed of the body of water on a particular side, rotation of the pipeline does not enable laying special parts in the correct configuration desired.
To overcome this drawback in at least intermediate waters, a float is connected to the special part to provide a force that maintains the desired orientation of the special part.
In deep waters, the configuration of the pipeline is nearly vertical and, in consequence, the action of the float is ineffective as the float is practically parallel to the pipeline. In this case, the float would be able to perform a corrective action only close to the bed of the body of water where the pipeline progressively assumes a horizontal configuration.
However, the action of the float limited to this short section proves to be insufficient.
The use of a float would also be impractical for correcting rotation of the special part when the rotation starts outside the body of water and is induced by plasticization.
Great Britain Patent No. GB 2,460,671 and Great Britain Patent No. 2,462,656 teaches the release torsional strain from a pipeline at a joining station.